The present invention relates to a powderous lithium transition metal oxide, containing a special type of Mn and Ni bearing LiCoO2. The cathode powder can be prepared at large scale by a low-cost process. More specifically, the preparation is the sintering of a mixture of a cobalt containing precursor, like LiCoO2, a Ni—Mn—Co containing precursor, like mixed hydroxide MOOH, and Li2CO3. The sintering temperature is high enough to allow for an exchange of cations between the LiCoO2 and Li—Ni—Mn—Co oxide phases being formed, which results in a very specific morphology with a compositional gradient of the different transition metals. The lithium transition metal oxide powder can be used as a cathode active material in rechargeable lithium batteries.
Despite of some inherent limitations like poor safety and high cost LiCoO2 still is the most applied cathode material for rechargeable lithium batteries. There is a strong demand driven by customer expectation to increase the energy density of rechargeable lithium batteries. One way to improve the energy density is to increase the charge voltage, which requires more robust cathode materials which can be charged at higher voltage. Problems which appear or become more severe if the charging voltage is increased are (a) low safety, (b) poor storage properties during storage of charged batteries at elevated temperature and (c) poor cycling stability. Numerous approaches have been disclosed to address these problems. Partial improvements have been achieved but the basic problems have not been fully resolved.
The characterization of phase transitions during charging-discharging, hence as a function of x in LixCoO2, has played a central role in the study of this material, as phase changes can introduce irreversibility with repeated charge and discharge. Several investigations have identified phase transitions in layered LixCoO2 above 4.3 V. LiCoO2 is isostructural with the rhombohedral R-3m α-LiFeO2 and is referred to as “O3”. The O3 structure can be thought of as an ordered rock salt with an oxygen close packing sequence ACBACB and the Co and Li ions forming CoO2 and LiO2 planes of edge-shared octahedra alternately ordered in the (111) direction. When Li is removed a two phase region is observed when x is less than about 0.75. The driving force of the two phase domain is thought to be a Mott-insulating transition from localized spin-holes to metallic-like conductivity when x˜0.75. At x=0.5 and V˜4.15V, a monoclinic transition occurs driven by Li/vacancy ordering and charge ordering within the CoO2-planes. When more Li is deintercalated; LixCoO2 undergoes a cascade of first-order phase transitions with the appearance of a monoclinic H1,3 phase near 4.55V (x˜0.2) and eventually the formation of a fully delithiated hexagonal O1 CoO2 phase with ABAB oxygen packing sequence at 4.62V. See for example A. Van der Ven, M. K. Aydinol, and G. Ceder, in J. Electrochem. Soc., 145, 2149 (1998). Such structural transitions at high voltage from O3, H1,3 and O1 induce CoO2 plane-gliding which can eventually result in structural instability during repeated charge and discharge cycling and could be responsible for the poor performances of LiCoO2 in real commercial cells at higher voltages. In particular, several research groups have reported multiple failure mechanism of LiCoO2 at high voltage including impedance growth in the cell, resulting from side reactions involving LiPF6-based electrolytes and initial surface degradation of LiCoO2 particles caused by air or moisture exposure, cobalt leaching and elution, possibly assisted by by-produced HF attack, formation of cubic spinel phase at the LixCoO2 particles/electrolyte interface and an increase of dislocations and internal particle strains.
Beside the demand to increase the energy density, it is essential that rechargeable batteries meet the power requirements. That means that the battery as a whole and particularly the active cathode material itself has a sufficient high rate performance. Careful studying of published results on cathode materials allows to better understand the limitations of LiCoO2 based rechargeable lithium batteries. One basic limitation originates from the surface area dilemma. Increasing the rate performance (i.e. high power) can be met by increasing the surface area because the solid-state lithium diffusion length can be decreased; which results in an improved rate performance. However, a high surface area increases the area where unwanted side reactions between electrolyte and charged cathode take place. These side reactions are the cause of poor safety, poor cycling stability at elevated voltage and poor storage properties of charged cathodes at elevated temperature. Furthermore, high surface area materials tend to have a low packing density which reduces the volumetric energy density. Another basic limitation originates from the cobalt stoichiometry. Lithium-nickel-manganese-cobalt oxide based cathode materials (like LiMn1/3Ni1/3Co1/3O2) have higher stability against reactions between electrolyte and cathode than LiCoO2, and the raw material cost is lower, but these materials suffer from a lower volumetric energy density and these materials typically have a lower lithium diffusion constant.
It can be concluded that there exist basic limitations in:                surface area: Low surface area cathode materials are desired to achieve high safety, improved density and high stability during storage; however, the surface area cannot be lowered too much because this will lower the rate performance, and        composition: LiMO2 cathodes, where M dominantly is cobalt is desired to achieve high lithium diffusion rate and high volumetric energy density; however a high content of cobalt causes poor safety properties, increased cost and an inferior high voltage stability.        
A solution to this dilemma would be to increase the diffusion constant D. A higher value of D would allow to lower the surface area without losing rate performance.
LiMO2, where M=Ni—Mn—Co with Ni:Mn>1, has been previously disclosed. U.S. Pat. No. 6,040,090 (Sanyo), for example, discloses a wide range of compositions including LiMO2 with Ni:Mn>1. The patent application discloses that LiMO2 has a high degree of crystallinity (small FWHM of peaks in the X-ray diffraction pattern). LiCoO2 doped with Ni and Mn has for example been disclosed in U.S. Pat. No. 7,078,128.
It is an object of the present invention to define a cathode material having a high rate performance, and showing high stability during extended cycling at high charge voltage. The high temperature storage properties are also improved.